In general, a mask pattern formation technique employed in fabricating a semiconductor device has a significant effect on accuracy of a pattern formed on the semiconductor device. Particularly, if an optical proximity effect of a mask pattern is not properly considered, distortion of a pattern line width, such as shortening of linearity of the line width, occurs contrary to the original purpose of lithographic exposure, which results in deterioration of characteristics of the semiconductor device.
On the other hand, a semiconductor photolithography technique can properly adjust the amount of light passing through a mask by designing the mask elaborately. To this end, an optical proximity correction (OPC) technique and a phase shifting mask technique have been introduced and various methods for minimizing light distortion due to deformation of a mask pattern have been used.
Recently, with the development of chemically amplified resists, which are very sensitive to light having a far-ultraviolet wavelength of 248 nm or 194 nm, practical techniques for enhancing resolution have been introduced. Particularly, a technique for forming an auxiliary pattern, which is separated from a main pattern, such as a dummy pattern used to control an optical proximity effect, contributes to enhancement of resolution.
FIGS. 1a and 1b are diagrams showing the original of a semiconductor mask pattern and a mask pattern having a fine auxiliary pattern and to which the well-known OPC technique is applied. Referring to FIG. 1a, the conventional semiconductor mask pattern includes a gate pattern 1 as a main pattern of a logic device, which corresponds to a light shielding region. Other portions in the mask pattern correspond to a light transmitting region 2. FIG. 1b is a diagram showing an OPC pattern attached to the main pattern 1 to alleviate non-uniformity of light intensity distribution, and a fine auxiliary pattern 1c disposed in the vicinity of the main pattern. The fine auxiliary pattern 1c refers to a fine pattern having a line width less than a resolution limit. Although this fine auxiliary pattern 1c exists in the mask, it is not present on a semiconductor substrate after an exposure process is performed.
Resolution of the division patterns can be defined by a Rayleigh's equation as expressed by Equation 1.R(Resolution)=k*λ/N.A.  Equation 1
Where, k is constant, λ is a wavelength of light emitted from an illuminometer, and N.A. is an aperture of an illuminating lens. For example, when k is 0.5, λ is 0.248, and N.A. is 0.65, resolution (R)=0.19 μm. Accordingly, when a fine pattern having a line width less than the value of the resolution is independently applied to a mask, a pattern for passing light through only the mask physically while an image is not formed in a photosensitive agent can be defined.
Referring again to FIG. 1b, the fine auxiliary pattern 1c has problems in resolution of edges of a pattern. Particularly, if the fine auxiliary pattern 1c contacts with another auxiliary pattern 1c at a certain angle, as indicated by reference letter A, or is not properly isolated therefrom, as indicated by reference letter B, there is a problem of pattern distortion on the semiconductor substrate.
FIGS. 2a to 2c are diagrams that depict known mask pattern formation problems. FIG. 2a shows a mask including a plurality of fine auxiliary patterns, which are attached to each other. FIG. 2b shows overlapped contour images having a light intensity distribution depending on the amount of exposure energy of light passing through a mask, and FIG. 2c shows a continuous distribution of light intensity in the vicinity of edges of a main pattern with an angle of 90 degrees.
FIG. 2a shows an enlarged mask corresponding to the region A in FIG. 1b. As shown in FIG. 2a, a plurality of fine auxiliary patterns 1c are attached to each other, as indicated by reference numeral 10a, in the vicinity of edges (with an angle of 90 degrees) of a main pattern 1. The main purpose of arranging the fine auxiliary patterns is to enhance resolution. The fine auxiliary patterns must not be formed on a semiconductor substrate. However, because the intensity of light at a portion where the fine auxiliary patterns 1c are attached to each other is very low if lack of exposure is significant or an optical apparatus has an unstable focus depth, undesired traces of the auxiliary patterns appear on a resist pattern of the semiconductor substrate. These traces become contamination sources by particles when an etching process is performed. FIG. 2b shows overlapped contour images 3a and 3b having a light intensity distribution depending on amount of exposure energy of light passing through a mask. As shown in the figure, as an optimal exposure distribution 3a is changed to a lack of exposure distribution 3b, the distribution of light intensity in the vicinity of the edges (with an angle of 90 degrees) of the main pattern 1 is seriously distorted and the edges to be formed on the semiconductor are rounded.
FIG. 2c shows a continuous distribution of light intensity in the vicinity of edges of a main pattern with an angle of 90 degrees. In FIG. 2c, a region with a dark color represents a portion showing weak light intensity and a region with a light color represents a portion showing strong light intensity. Reference numeral 20a denotes a pattern formed by an optimal exposure. Here, the optimal exposure refers to energy conditions under which exposure apparatus emits light such that a pattern has the same line width as the original mask shown in FIG. 1a. FIG. 2c shows a distribution having a problem in that the edges are rounded.
FIGS. 3a to 3c are diagrams for explaining problems when a mask pattern is formed in the prior art. In particular, FIG. 3a shows a mask including a plurality of fine auxiliary patterns, which are isolated from each other, FIG. 3b shows overlapped contour images having a light intensity distribution depending on the amount of exposure energy of light passing through a mask, and FIG. 3c shows a continuous distribution of light intensity in the vicinity of edges of a main pattern with an angle of 90 degrees.
FIG. 3a shows an enlarged mask including a plurality of fine auxiliary patterns 1c, which are isolated from each other. As shown in FIG. 3a, the plurality of fine auxiliary patterns 1c are isolated from each other, as indicated by reference numeral 10b, in the vicinity of edges (with an angle of 90 degree) of a main pattern 1. FIG. 3b shows overlapped contour images 3a and 3b having a light intensity distribution depending on amount of exposure energy of light passing through the mask. As shown in the figure, as an optimal exposure distribution 3a is changed to a lack of exposure distribution 3b, the distribution of light intensity in the vicinity of the edges (with an angle of 90 degree) of the main pattern 1 is seriously distorted and the edges to be formed on the semiconductor are rounded. Particularly, a pattern distortion at the edges with an angle of 90 degrees causes a more serious problem of a process margin for alignment of a contact hole mask, compared to a pattern distortion at the edges with an angle of 270 degrees.
FIG. 3c shows a continuous distribution of light intensity in the vicinity of edges of a main pattern with an angle of 90 degree. In the figure, reference numeral 20b denotes a pattern formed by an optimal exposure.